In the seismic application of LOFAR, the aim is to continuously monitor the subsurface with seismic-vibration sensors. The vibrations may have natural or human causes. Natural causes can be wind or earth tremors, while human causes can be traffic, building activities or deliberate vibrations as commonly used in seismic exploration. From such seismic recording we can:
The first two themes are the realm of the more globally oriented seismologists while the third theme relates to the more locally oriented exploration seismologists. In the Seismic Application of LOFAR a contribution to all these themes will be made. The common denominator between these themes is related to the specific situation in the north of the Netherlands, namely the gas fields and the associated earth tremors due to the gas-production. In the first theme, the earth tremors are used for obtaining information of the deeper subsurface; in the second one, more will be understood about these earth tremors themselves; and in the third one, the changes over time above those gas fields will be revealed.
Apart from revealing the subsurface and its changes over time, the seismic application of LOFAR also addresses technological issues. Very recently a new exciting technique has emerged, called Seismic Interferometry. It allows obtaining images of the subsurface without the use of deliberate vibrations as commonly used in seismic exploration. Seismic Interferometry can revolutionize the way in which seismic exploration is carried out. The LOFAR program pays special attention to the development of this technique. Also here a relation to the specific situation in the north of the Netherlands exists, since the “noise” from earth tremors may be exploited for such use.
Figure 1. Structures of the Netherlands, with main faults (red lines), contours of depth of bottom of the crust (white contours) and earthquake locations (white circles) (Source: Vrije Universiteit).